The object of the proposed research is to discover the basic cellular mechanisms that permit the retinal pigment epithelium (RPE) to phagocytize and digest the terminal disks of the rod outer segments (ROS). Utilizing our recently developed organ cultured RPE explant as a model system for phagocytosis, we will test young and senile, macular and peripheral human RPE for their relative ability to phagocytize inert particles. The effects of ultrastructural abnormalities of Bruch's membrane on phagocytic performance will be assessed. Bovine RPE explants will be used to test specific steps in acute phagocytosis: recognition at the cell surface, the role of microtubules and microfilaments in engulfing movements, changes in organellar and cellular volumes, and possible shifts in synthetic activities. The digestive process will be studied by feeding RPE radiolabeled biological particles, bacteria, ROS, etc. and determining the rate and pattern of breakdown of macromolecules by RPE lysosomal enzymes. Bovine RPE will be fractionated and the enzymes of the subcellular organelles will be further characterized. Particular attention will be given to peroxidase, superoxide dismutase and other enzymes involved in protection against free radical damage.